Inorganic particles that are sensitive to atmospheric materials such as moisture can be coated using liquid based or vapor phase methods. For example, binders and mineral or metallic colloids can be dispersed in liquid media and distributed over the surface of the inorganic particles to be coated. However, the coating materials must be insoluble in, and non-reactive with, the solvent used. Vapor phase coating methods, such as using fluidized bed reactors, require high temperatures and lengthy reaction times which generally result in some surface reaction or modification of the particles to be coated. Further, the equipment is complex and expensive.
As an example, long persistence phosphors are known which, after having been struck by light, continue to emit light after the initial light source is extinguished. For example, europium activated alkaline earth metal sulfides, such as SrS:Eu and CaS:Eu, are known that emit in the orange-red portions of the spectrum. Red-emitting phosphors having long persistence are very rare, but they are very useful, particularly for safety equipment such as xe2x80x9cEXITxe2x80x9d signs and the like.
However, these phosphors are water sensitive, and thus cannot be exposed to moisture without deleterious effects on their light emission. Thus they must be protected to prevent exposure to moisture.
Coating or encapsulating water sensitive phosphors with moisture insoluble materials has been tried. The coating particles can be mixed with the phosphor particles to be coated in a liquid medium, but both the coating material and the phosphor must be insoluble in, and non-reactive with, the liquid. In the case of phosphors, moisture sensitive phosphors cannot be immersed in solutions including any moisture.
Vapor phase coating has also been tried; for example tetraethoxysilane or tetramethoxysilane can be passed through a fluidized bed including phosphor particles to deposit a silica layer, but this is an expensive process.
Thus it would be desirable to be able to apply a coating of an inorganic material onto inorganic particles to form a protective coating, or a coating that can modify particular surface properties, or impart a particular surface functionality to the inorganic particles.
In particular, a method of applying a water impervious coating onto phosphor particles that are sensitive to moisture without the aid of an aqueous liquid medium, a method that does not change the color or the strength of the phosphor emission, would be highly desirable.
The present invention is directed to a method of coating inorganic particles with a thin film of an inorganic coating material that does not require any liquid media for its application. Inorganic particles having a particle size of from 1 up to about 100 micrometers can be coated using a sufficient amount of nanometer sized inorganic powders to completely cover the larger inorganic particles.
The coated inorganic particles are then fired at a temperature high enough to soften or melt the nanometer-sized coating particles surrounding the coated inorganic particles, but a temperature insufficient to change the surface properties of the larger inorganic particles to be coated. The present method does not require that any solvent be used.